Mail gone from working perfect to extremely glitchy? Anyone else?

Question

Just wondering if there has been an update glitch or something. I am running two computers, one G4 silverdoor tower and a MBP. Mail has worked flawlessly for the time I have owned both computers. Now, they both seem to glitch constantly!
- mail won't send, does not recognize STMP. Will sit in outbox, then randomly decide everything is ok now? No pattern to behaviour.
- mail will send, but never actually get go to intended person. Randomly. Some will, some won't. Even if they are sent seconds apart from each other.
Just wondering if anyone else has run into similar problems. I have contacted my hosting provider and they have tested everything from there end and have not found a problem? Not sure why it is an issue now? Everything worked perfectly before, and I have not changed anything.
Any suggestions
TY

Answer 1

If this email account and SMTP server used to send messages with the account is provided by your hosting provider which is not your internet service provider used for connecting to the internet, any problems with messages being sent are likely something you need to address with your ISP.
Is your hosting provider's SMTP server authenticated?
Are you using 25 for the Server Port with this SMTP server?
If so, try entering 587 in place of 25 in the Server Port field for this SMTP server - Mail > Preferences > Accounts under the Advanced tab for the account preferences - Server Settings button for this SMTP server.
Are you also access an email account and SMTP server that is provided by your ISP with the Mail.app and if so, any problems sending messages with this account?
A successfully sent message (a message moved to the account's Sent mailbox is an indication the message was successfully sent) is either accepted or rejected by the recipient's incoming mail server. Most of the time you will receive a return (bounced) email error message from the recipient's incoming mail server if a message is rejected by the server for whatever reason but not always - especially with the amount of spam being circulated today with spammers using what is called a dictionary attack on an ISP's or email account provider's domain.
If a successfully sent message is not received by the recipient, this is not a Mail.app problem. This problem more than likely is at the recipient's incoming mail server. Your email address or the domain for your email address or your ISP may have been blacklisted by the recipient's incoming mail server if they believe spam is or has been emanating from the provider's domain or from your ISP's domain. A computer's IP address can be blocked or a range of IP addresses by a recipient's incoming mail server.