Who is the mail provider, and what is their size limit? Note that message encoding adds about 33% to the size of attachments, so a message with 7.5MB of attachments results in a message size of 10MB.

Mail provider is Spectrum. I've sent many messages of 20MB or larger to the same group (size ~50 members). Something has changed recently, just not sure what or where to start looking. Maybe I need to ask Spectrum next?

Spectrum states the size limit is 25MB, which means attachments should be no larger than about 18MB. Check also that you aren't near the quota for mailboxes (5GB), which includes sent messages.

Good thought on the "sent mail", but I have less that 145MB out of 5GB used. Just to be sure, I deleted all sent mail and emptied the trash on the Spectrum website. Still no joy, same error message. Interestingly enough, the message does show up in my Sent Mail Folder on Thunderbird?? Shows a size of 8.8MB. The message shows up again each time I try to send it.

Is the error coming from your server or the recipients' server? If the latter, you may be exceeding a different size limit, which you can check on their website.

I think it's coming from my server. When I hit Send, I get a pop up window saying "connecting to SMPT server roadrunner.com". After a few seconds, that window goes away, I get an error tone and another window pops up labeled "Alert", which contains the following text: The size of the message you are trying to send exceeds a temporary size limit of the server. The message was not sent; try to reduce the message size or wait some time and try again. The server responded: 4.1.1 VjJBiFrRsWkjHVjJBiokPJ internal error AUP#CMPOL. The only option is the "OK" box. Click that and the Alert window goes away, the message shows up in the Sent Folder. Weird, since I've been doing this same thing 3 days a week for several years and never had a problem!

It may have something to do with number of emails you send in one go.

Have you tried sending to 25 people? Try splitting up the email into two batches of 25 instead of one batch of 50.

I have considered that, thanks. Spent a couple of hours online with Spectrum Help yesterday...his conclusion was that Spectrum is having "temporary" problems with their servers, hence the "temporary" size limitation (which didn't exist a week ago). Will continue trying and see if the size limit goes away sooner, rather than later. Thanks for your help.

Size limits can be set to Zero if you do not authenticate correctly. Can you send mail using that server at all? say a small email to another of your mail accounts. that would exclude authentication issues and most anti virus interactions, although if it is all mail with attachments, anti virus have caused issues with sending mail in the past.

I would guess, as you are using a roadrunner server that is will be set to limits possibly imposed back in the days when road runner was running the show. Don't spectrum have a smtp server of theirs that you can use to send mail?

I called Spectrum today. When I send message using Spectrum's email website, no problem. When I use TB, get the following response: "The size of the message you are trying to send exceeds a temporary size limit of the server. The message was not sent; try to reduce the message size or wait some time and try again. The server responded: 452 4.1.0 <…….@charter.net> server temporarily unavailable. AUP#O-1380."

As suspected, Spectrum had problems with their server, which took them about a week to solve. Once they fixed the server, I have no more problems. Thanks all.