email client software = TB = Thunderbird, etc.

if a user is using a commercial/business type internet connection ( obtained from ISP,etc ) , then that type of user need to use port-25 (SMTP) in their email client software, to send email out (into a remote mail-server) via their own mail-server. Commercial/business type account DOES-NOT have PORT-25-BLOCKED/restriction , so these users can use that outgoing+incoming port. usually commercial/business type account has fixed (routable) IP-address.

business type account connection clients/users can also use port 465 or 587 (beside port 25) to submit outbound email to their mail-server, then their mail-server sends it out to destination mail-server.

if a user is using a residential/home type internet connection, then that type of user need to use port-465 or port-587 in their email client software, to submit emails first into their mail-server , then to send the emial, their mail-server uses port-25 to connect with remote+destination mail-server. in residential type account, outgoing PORT-25 is BLOCKED/restricted, (and often inbound port-25 is also blocked). usually residential/home type account has dynamic or non-fixed (routable) IP-address.

ISP = internet service provider. MSP = mail service provider.

some ISP/MSP allots port 465 to their clients/members, to use as SMTP (to send outbound email). some ISP/MSP allots port 587 to their clients/members, to use as SMTP.

if the security/encryption protocol ISP/MSP decided to use is STARTTLS for SMTP, then it is not-good. if the security/encryption protocol ISP/MSP decided to use is TLS/SSL for SMTP, then it is good.

because, TLS is much much more secured than StartTLS. there are many EXPOSED records+news of spying (stealing/theft of user's private data) via StartTLS. if your ISP/MSP uses StartTLS, then PUSH them to use secure/safer TLS.

Some ISP MAY allow a residential type account to use outgoing PORT-25, when account-holder will call them to remove that restriction. ( you can request your ISP to allow both INCOMING & OUTGOING port 25 (aka SMTP port) , that may/will allow you to connect with (your own or your office's) remote mail-server over port-25 directly. ) ( if you want to use/create your own PERSONAL (NOT-BUSINESS) mail-server inside your own home : then 1st request your ISP to allot you a fixed IP-address, that may cost $1/month extra . then tell ISP to open/allow both incoming/outgoing port 25 , 80 , 443 , etc . then tell ISP to set a very specific rDNS dns recond (aka "PTR" dns record) for that fix-IP-address, lets say you need to setup rDNS: server1.example-my-domain-name.com. , so before you tell ISP to set rDNS you must have to purchase your own domain-name example-my-domain-name.com ( it may cost from $5 to $20 per year ) . Before purchasing domain-name call your ISP & find out what exact fee/cost are needed to get a fixed-IP-address, & can they setup the rDNS/PTR record for that fixed-IP-address with your domain-name or not . When your ISP will allot you fixed IP-address & sets-up the rDNS that you want , then you can setup your own mail-server inside your own home or home-office . Use any free server software/tools to build/make/configure your own mail-server. )

To view/get the received emails from a mail-server , user need to use either POP based protocol or IMAP based protocol , in their email client software. POP port 110 and IMAP port 143 , does not use Security/Encryption protocol , so these ports MUST not be used. So users must use POPS port 995, OR, IMAPS port 993 , these ports are allotted for Secured/Encrypted data usage , users MUST also select "TLS/SSL" or "TLS" or "SSL/TLS" as security/encryption in their email client software) , so users MUST use either IMAPS/IMAP4S or POPS/POP3S. often many documentation/webpage will use the word "POP" instead of "POPS", & use the word "IMAP" instead of "IMAPS", with configuration port 995 or 993 & with "TLS/SSL" encryption/security protocol, so its okay to use those words in such case. The last "S" in POPS/POP3S or in IMAPS or IMAP4S is indicating the word "Secured". if an email client or a webpage uses port 143 or 110 then they MUST not use the word IMAPS/IMAP4S or POPS/POP3S.

Hello Erik. Very many thanks for your extremely detailed response. Just to clarify the settings I am presently using. I am using incoming server port 110 with connection security STARTTLS. Outgoing server port currently 587 with connection security STARTTLS as well. In both case the " authentication method " is " normal password." I have to say that these settings have always worked fine ( with Thunderbird ) except for a few days ago when I suddenly got the message " too many E-mails " and then I altered the outgoing server port from 587 to 25. However, since then I have changed it back to 587 and everything is working fine again. Perhaps you would confirm whether the current settings should be left as they are ? Obviously reluctant to change anything while all seems to be OK. Incidentally, I am a residential / home user. Many thanks.

Usually, that type of error message indicates that a limit counter has been triggered. You usually experience a temporary stop on sending. That limit could number of sent messages within a given period of time eg: number per hour or per day.

Sometimes, the error code (though is unusual) may suggest that the mail sending is not being fully authenticated, but judging by your settings that is not the case.

You mention TT, so are you using TalkTalk ? If yes, then email settings are as shown at the link below: https://community.talktalk.co.uk/t5/Articles/Email-settings-IMAP-amp-POP3/ta-p/2204399

TalkTalk limits: Too many recipients is generic for a spam protection sending limit. Sending to more than 50 recipients per hour or more than 500 per day will trigger a limiter. But it can also be triggered by sending to too many invalid addresses. If the normal rate limiter has been triggered then it'll be reset in an hour or within a day depending on whether it's the hourly limiter or 24 hour limiter that's been triggered.

Hello again Toad Hall. I checked the link you sent me to Talk Talk and have printed off the various E-mail server settings. I can refer to them in future should I get any more problems. You will see from the above threads that I am advised not to use port 110 nor security encryption protocol STARTTLS. Apparently, neither of these settings is especially secure. However, after going through all the settings remotely with the technician at my local shop, he has confirmed the existing settings which are ( incoming port ) 110 with STARTTLS and authentication " normal password. The outgoing details are port 587 also with STARTTLS ( server name starts with SMTP ) and authentication the same " normal password." I am pleased to say that all seems to be working fine since he changed the server name for me. So not looking to make any further changes at this time. But at least I have the notes showing the correct server settings so I can refer to them in future. Thanks again for your help and support.

re :You will see from the above threads that I am advised not to use port 110 nor security encryption protocol STARTTLS. Apparently, neither of these settings is especially secure.

That's correct. Even talk talk provide server settings that use a secure connection.

re : all seems to be working fine since he changed the server name for me.

If your issue all this time was not using the correct mail server name eg: mail.talktalk.net, then the ports would not have worked anyway. So there is nothing stopping you from changing the port and connection security to those set out below so you are using a secure connection.

Incoming POP mail server mail.talktalk.net Incoming Port 995 Connection Security: SSL/TLS Authentication Method: Normal Password. User name : full email address.

Outgoing mail server smtp.talktalk.net Outgoing Port 587 Connection Security: SSL/TLS Authentication Method: Normal Password. User name : full email address.

Thanks Toad-Hall. You are right in that - apparently - the problem all along was that the mail server name was incorrect. However, the one I was using had worked fine for the past several years so I don't know why it suddenly started giving trouble. Perhaps Talk Talk had altered something with their last up-date. Anyway, all looks good now so I shall close this " thread " and hope things stay that way ! Thanks again and Merry Christmas !

Hello again Toad-Hall ! Having changed my E-mail settings as per the link you sent me from Talk Talk, am pleased to report that all seems to be working OK. Except that I have been unable to switch from STARTTLS ( as advised ) to SSL/TLS. On both my incoming and outgoing server connections I have changed the port settings as per your instructions, but the connection security settings always default back to STARTTLS the moment I enter the port numbers. Are you able to advise me in simple terms exactly what steps I need to take to change the connection security settings ? Or - since all seems to be working OK so far - should we just leave things as they are ? Many thanks.