Mail won't send from iMac under Optimum

Trying to help a friend who has an iMac (2010 I believe) and optimum internet. After some recent email service on optimum's end, her email stopped working. I got her account back online in Apple mail, however the outgoing mail server won't stay connected. Had chatted with optimum tech support who suggested setting up her account as an IMAP account (she had been using a POP account). Used all the settings they provided and have the IMAP account getting mail, but only sporadically allowing mail to be sent. What happens is re-enter her user name and password and the outgoing server will go online for about 20 seconds and then immediately goes offline. Sometimes when it goes online it will allow an email stuck in her outbox to send. However once it sends it goes immediately off line and can't get it back online unless I delete that server and set it up all over again.

Optimum tech support sucks, especially for Mac but did get one person who said they were having problems on their end and he was going to escalate it to an engineer.

My friend got a call today, not sure what level person, who said if she could send emails through her webmail account (which she can) then the issue is not with Optimum but with Apple Mail or her iMac. I believe it is an issue on Optimum's end, but 2 low-level people from Optimum say otherwise.

So, my question is, does this sound like an issue with her iMac or something on Optimum's end. If with her iMac, any suggestions?

Thanks