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My ISP has problems with their outgoing mail server, resulting in a significant portion of my outgoing emails being discarded on a more or less random basis. I'm using IMAP.

I also have another account with a different ISP. If I were still using good old POP3, I could use the second ISP's SMTP server as the outgoing mail server for both accounts, and use the first only for incoming mail.

Is this possible at all with IMAP? And if so, how do I set it up in Mozilla Thunderbird? (Google has failed to enlighten me!)

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    @LightnessRacesinOrbit No, you are wrong, from the point of MTA view, emails belonging to others ISP are aliens and shouldn't be processed if they don't want be banned – Alex Jul 21 '18 at 16:07
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    No, you're wrong :) "But why would they do this?" When usual 12345 super-duper user's passwords would be hacked/brute-forced, attacker will start sending emails like urgent@paypal.com and similar. While normal receiving server will reject it (and reported it !!!) because PayPal has SPF record, a bunch of shared hosting that don't even bother to check SPF will happily accept this forged email and in a few hours MTA that sending such stuff will be in all popular antispam databases and as result will blocked by all normal MTA in the world. – Alex Jul 21 '18 at 20:12
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    Second, any email provider (if they care about reputation) will set in their DNS SPF record that tell all other servers in the world - which MTA (MX or IP...) is only one who responsible to send emails for particular domain. Another reason - is DKIM that signing outgoing emails and prevent forging won't be happy to sign ALIEN email since public key in DNS won't match. And finally - why one email provider should do a job (including expenses) that must be done by another? BTW, would you personally trust email if you receive email from Gates@microsoft.com that comes from comcast.net server? – Alex Jul 21 '18 at 20:13
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    @FrankvanWensveen Before you going to start using ISP2 as outgoing server, check ISP1 if it allowing to do so by running nslookup -type=txt domainPartOfYour_ISP1_Email.com and look for record that start with "v=spf1". If such records exist and there no any information in such record regarding ISP2 then your emails would be tagged on receiving email servers as forged. – Alex Jul 22 '18 at 15:34
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    @FrankvanWensveen: What your second ISP allows isn't the problem. The problem is that the first ISP likely won't allow the 2nd one's SMTP server to impersonate it. – grawity Jul 22 '18 at 16:39
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It's possible. The two protocols are completely unrelated. IMAP doesn't care how you send mail, and SMTP doesn't care how you receive mail. In Thunderbird, the SMTP server settings work the same and should even be located in the same place.

The real problem is that many domains use SPF to define a small whitelist of which SMTP servers are allowed to send mail as that domain. So If your ISP1's domain uses SPF, and you try to send mail via ISP2's server, then most of your recipients will give it a high "spam" score due to SPF checks failing.

In other words, your rejection rates might even increase.

  • Regarding - "The real problem is that many domains use SPF". It is a problem only for OP and spammers, so it isn't a problem, but opposite - a good things. – Alex Jul 21 '18 at 16:13
  • Aha. I was looking under 'server settings' to change the server. My bad. This solves my problem! Thanks a mil! – Frank van Wensveen Jul 22 '18 at 15:23

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